The present invention relates to a battery saving circuit for use in a communication apparatus, such as a portable pager receiver.
In communication apparatuses including mobile radio units, such as radio paging receivers, there is used the so-called battery saving system, which periodically supplies and cuts off power to their receiver sections with a view to saving power consumption.
For instance, a transmitted signal in a paging system contains a preamble signal preceding a plurality of subscriber identification (ID) signals so that the receiver can be placed in a battery saving mode. The receiver, which periodically turns the power supply on and off, receives signals during each time period (Ton.sub.1) that the power is on, and does not receive signals during each time period (Toff) that the power is off. If a preamble signal is detected during Ton.sub.1, the ensuing time period of "power on" (Ton.sub.2) will be extended to receive the subscriber ID signal that is to follow.
In this system, since the preamble signal is detected only once, erroneous operation may be caused by noise or other unwanted signal, resulting in low reliability. Moreover, the erroneous detection of the preamble signal deteriorates the battery saving efficiency because erroneous detection unnecessarily extends the power-on time period (Ton.sub.1).
One example of a battery saving system to eliminate the foregoing disadvantages is disclosed in U.S. Pat. No. 4,181,893, issued to E. L. Ehmke on Jan. 1, 1980. According to this battery saving system, the receiver, in order to detect the ID signal, extends power supply to the receiver section in response to multiple detections of the preamble signal. This multi-detection prevents the receiver from causing erroneous operation, thereby improving the battery saving efficiency.
To carry out the first detection of the preamble, however, this battery saving system supplies power to the receiver section for a prescribed period of time regardless of the result of the first detection of the preamble signal. Therefore, this system yet has a disadvantage of lower battery saving efficiency.